In metallurgy, different processes have been developed for degassing metallic melts, after it was discovered that a high gas content affects disadvantageously the properties of alloys.
Many processes are based on the principle that the melt is brought into a vacuum-tight chamber, and there subjected to a vacuum. These processes required pumping systems with high vacuum power requirements, as well as vacuum-tight chambers. In view of the high temperatures of the mlts, such equipment is costly and gives rise to difficulties.
Other processes reduce the gas content by passing the melt over porous scavenging elements which may be stone-shaped where they are scavenged or cleansed with finely distributed inert scavenging gas, as for example, nitrogen or argon. Also belonging to the group of scavenging or cleansing gas processes are processes using reaction materials which are immersed in the melt, and are materials as, for example, polytetrafluorides. When these materials contact the melt, gases and vapors are generated which degas the melt in a manner similar to that achieved with scavenging gases.
The scavenging processes are substantially simple from the apparatus or equipment point of view. However, their degassing results are not satisfactory or sufficient in many cases. This often causes the requirement that several scavenging gas units may be applied simultaneously, and long degassing times be incurred. This leads to strong cooling of the melt and insufficient degassing at the top of the melt.
In another method, there is simultaneous processing of the melt with vacuum and scavenging gas. This method has the advantage that substantially smaller amounts of scavenging gas are required for treating the melt, whereby the degassing effect is increased.
In practice, these processes are often too expensive, since vacuum-tight processing equipment is required, on the one hand, and in view of the hydrostatic pressure of the melt, the scavenging gas must be introduced into the melt with high pressure, on the other hand, even though a vacuum prevails above the melt. This is against the effect for fine blowing.
When in scavenging gas processes lances are not generally used, then substantially high porous stones or elements are used which become easily clogged upon making contact with the melt. Such high porous elements also require a high gas pressure so that the gas can be finely distributed in the melt. As a result, the pressure of the scavenging gas is also not held substantially low in vacuum scavenging processes, as one might desire. In this particular aspect, the process in accordance with the present invention, provides advantages which are further described below.
A process is also known in the art in which a rotating member is immersed in the melt. The rotating member is constructed so that when subjected to sufficiently high rotation, a vacuum is maintained with respect to the melt, in the interior of the rotating member. This interior space of the rotating member is directly connected to a vacuum pump system, by way of a hollow axle. The advantage of this process is that a melt can be subjected to vacuum, without having to bring the melt into a vacuum chamber. The degassing effect which such a rotating member provides, is substantially large in melts as, for example, aluminum melts, since the rotation provides intensive stirring, as well as disruption of the new surfaces. These effects are exclusive of the vacuum. The severe breakup of the surface of the melt, results in a substantially rapid and complete gas emission. The removal of the extracted gases takes the direction of the center point of the rotational member, and is finally removed by suction through the hollow rotational axle by means of a vacuum pump system. The disadvantage of this process is that it is difficult to conduct the gases removed by suction, through the rotational axle. Such conducting paths must possess a relatively large diameter, in order to apply well the suction effect of the vacuum pumps to the melt. This then also causes the rotational connection to the pumps to be complex. A further disadvantage of this process is that in all cases where the melt possesses vaporized portions, as for example, manganese, zinc, etc., the conducting paths become slowly contaminated and are finally closed off.
A further and considerably significant disadvantage of the process known in the art is that the vacuum pumps will suck in the hot metallic melt when the drive for the rotating member becomes inoperative upon insufficient barometric pressure of the suction line system. This results in the destruction of the equipment, and fires as well as explosions may occur. Furthermore, it is necessary to take into consideration the factor that the pumping action of the rotating member in addition to the hydrostatic pressure of the metallic melt must also exceed the pressure difference of the vacuum pump. This condition requires a higher surface velocity of the rotating member. This requires a substantially greater mechanical apparatus and causes greater wear of the rotating member, as well as an unnecessarily high suction effect on the surface of the melt. Severe stirring motions are also not permissible for certain metals and alloys from the metallurgy point of view.